A metal nanoparticle colloid is a colloid having metal nanoparticles dispersed therein. Metal nanoparticles in a bare state hardly keep themselves stable physically and chemically, and they have remarkable chemical reactivity. Therefore, the metal nanoparticles are handled in the form of a colloid which is formed by covering the surfaces of the metal nanoparticles with a ligand, such as a surfactant, to stabilize them, and then dissolving the resultant nanoparticles in a solvent, such as an oil.
Conventionally, in the production technique of metal nanoparticles for use in a metal nanoparticle colloid, there have been made vigorous studies with a view toward developing metal nanoparticles having a smaller and uniform size, but, in recent years, the development of a technique for controlling the fine particles in their shape to form a metal nanoparticle colloid having metal nanoparticles of an anisotropic shape is desired.
The reason for this is, for example, that the metal nanoparticles of an anisotropic shape can be increased in the surface area, and hence can be improved in the effect of a catalytic reaction on their surfaces. However, the size of the metal nanoparticles is too small to form a metal nanoparticle colloid having metal nanoparticles of an anisotropic shape.
Patent documents 1 and 2 (shown below) disclose a method for producing a metal nanoparticle colloid rationally with high efficiency by an active liquid surface continuous vacuum deposition method. The active liquid surface continuous vacuum deposition method is a method in which a metal or alloy in a bulk form is heated and evaporated in a vacuum and the resultant metal atoms are allowed to adsorb on the surface of a liquid solvent, such as an oil, having the surface covered with surfactant molecules so that fine particles of the metal are generated on the surface of the liquid medium, and the resultant medium is collected.
In this method, a layer of the surfactant molecules has a role in effectively capturing the flying metal atoms, and further has a role in covering the surfaces of the metal nanoparticles immediately after generated by condensation of the metal atoms to make the metal nanoparticles miscible with a solvent, such as an oil, and dissolve them in the solvent. Furthermore, the surfactant covering layer has a role of barrier in preventing the unstable metal nanoparticles from colliding with one another and fusing together and growing into larger particles or metal bulk.
By conducting this process in a vacuum container provided in a rotating drum in a continuous manner, a colloid system having nanoparticles having a uniform size dispersed in an oil solvent at a high concentration is formed. In this method, a metal or alloy fine particle colloid having the smallest and uniform size can be easily obtained using a simple apparatus, and further the method can be applied to various types of metals and alloys.
In the active liquid surface continuous vacuum deposition method, a ferromagnetic metal nanoparticle colloid used in a magnetic fluid or the like is easy to produce; however, a non-magnetic metal nanoparticle colloid is difficult to form. In addition, even in this method, a metal nanoparticle colloid having metal nanoparticles of an anisotropic shape is difficult to obtain.